1. Field of the Invention
This disclosure relates generally to methods for the destruction of unwanted byproducts in tail gas treating units, and more particularly, to methods for the destruction of byproduct ammonia gas in Claus tail gas treating units associated with Claus reactor units.
2. Description of the Related Art
The “modified Claus” process is commonly employed for the recovery of elemental sulfur from byproduct hydrogen sulfide (H2S) gas produced in various industrial sectors, most notably natural gas production and petroleum refining. This recovered elemental sulfur may be used in a variety of applications, including the production of sulfuric acid. Conversion from byproduct hydrogen sulfide generally begins with a controlled combustion with oxygen (O2), typically as air, in which approximately ⅓ of the H2S is oxidized to sulfur dioxide (SO2), as illustrated in equation (1):H2S+3/2O2→SO2+H2O+ΔH [combustion]  (1)The SO2 so generated then reacts with the remaining hydrogen sulfide (H2S) to form sulfur and water vapor according to the Claus reaction as shown in equation (2):2H2S+SO2→3S+2H2O+ΔH  (2)
In summation, the net overall reaction for the recovery of elemental sulfur from by-product hydrogen sulfide (H2S) may be described as shown in equation (3), below.
Roughly half of the sulfur produced in such a manner is formed in the reaction furnace, with the balance typically being generated by continuation of reaction (2) in one or more downstream catalytic stages.3H2S+3/2O2→3S+3H2O+ΔH  (3)
The refining processes conducive to formation of byproduct H2S from more complex organic sulfur compounds also tend to convert nitrogen compounds to ammonia (NH3). Subsequent recovery of the NH3 will typically yield a roughly equimolar gaseous mixture of NH3 and H2S (and water), which usually can be conveniently fed to the Claus reaction furnace in combination with a larger H2S stream containing negligible NH3. The NH3 is ostensibly oxidized as shown in equation (4).2NH3+3/2O2→N2+3H2O+ΔH  (4)
However, since the oxygen (O2) used is nominally limited to that required for oxidation of only ⅓ of the hydrogen sulfide, the hydrogen sulfide and ammonia are in competition for the available O2 such that initial SO2 generation likely exceeds that required by the Claus stoichiometry, and much of the ammonia is subsequently oxidized to generate nitrogen gas and water by reaction with SO2 as shown in equation (5).4NH3+3SO2→2N2+6H2O+1.5S2 [conversion]  (5)
In Claus reactions, exactly one third of the feed sulfur must be oxidized to SO2, requiring close control of the supply of oxygen in excess of that consumed by other reactions, including the combustion of hydrocarbons and ammonia. Fluctuations in the rate of ammonia addition into the feed may therefore have an unwanted, disproportionate effect upon the oxygen demand. Clearly then, the amount of vapors containing ammonia and hydrogen sulfide which can be treated in this manner is limited, and it is generally accepted that the maximum amount of ammonia (NH3) that can be so processed is about 30-35% of the total Claus feed, on a wet basis. Several processes for the treatment of vapors containing ammonia and hydrogen sulfide so as to address these issues and limitations are known and have been described in the art.
For example, a unique furnace design, which permits the combustion of proportionately greater amounts of NH3, has been described in U.S. Pat. No. 5,904,910. This patent suggests a method for producing hydrogen and sulfur from a first gaseous mixture containing hydrogen sulfide and ammonia by separating ammonia from the first gaseous mixture to produce a second gaseous mixture containing hydrogen sulfide; combusting a portion of the hydrogen sulfide in the second gaseous mixture to produce a third gaseous mixture containing hydrogen sulfide and sulfur dioxide; heating the ammonia to a temperature of at least 1800° F. to produce a fourth gaseous mixture containing nitrogen and hydrogen; and, combining the third gaseous mixture and the fourth gaseous mixture and passing the combined gaseous mixture to a sulfur recovery process wherein the hydrogen sulfide and sulfur dioxide are recovered as sulfur. The ammonia may be partially oxidized by the use of substoichiometric amounts of oxygen or thermally dissociated. Other options which have been suggested for addressing this problem include conversion of the NH3-bearing stream to ammonium thiosulfate fertilizer, and purification of the NH3 for marketing as a byproduct or incineration.
Other approaches in addressing this problem have included the controlled injection of gas injections into a Claus furnace using parallel injections and injection speeds, using modified catalysts, using gas permeable membranes and specialized treatment solutions, and using sensors such as laser diodes at the outlet of a Claus furnace in order to control the flow rate of gasses and minimize the amount of oxygen-rich gas required by the process. More recently, approaches using specialized burner designs having specific tube orientations and diameters in order to at least partially oxidize gas streams comprising hydrogen sulfide and ammonia, such as described in U.S. Pat. No. 6,890,498 to Tsiava, et al.
In view of these problems with the production and control of ammonia during Claus productions, improved methods for the clean, safe and efficient destruction of ammonia in tail gas treating units are necessary. This application for patent discloses methods for the efficient and environmentally favorable treatment of vapors which contain ammonia for the destruction of the ammonia in Claus tail gas treating units.